1. Field of the Invention
The present invention relates both to a front-illuminated-type photodiode array in which a plurality of photodiode units for optical communication are unified and to a front-illuminated-type photodiode array for a sensor in which a plurality of photodiode units are arranged two- or one-dimensionally.
2. Description of the Background Art
The field of optical communication has been achieving widespread adoption of the wavelength division multiplexing (WDM) technology, which enables the transmission and reception of a plurality of optical signals having different wavelengths through a single optical fiber. After the separation of the wavelength-multiplexed optical signals, in order to receive individual optical signals having a different wavelength, a plurality of independent photodiodes (PDs) arranged in parallel are sometimes used. For the same purpose, a photodiode array in which a plurality of photodiode units are unified is also used.
In addition, for an optical sensor or image pickup device, a photodiode array is also used in which a plurality of photodiode units are arranged two- or one-dimensionally. In these cases, the suppression of the electrical and optical crosstalk between neighboring photodiode units to a low level is strongly required.
The structure of a conventional photodiode array is explained by referring to FIG. 1. An absorption layer 3, which is either undoped or low-doped with an n-type dopant, and an n-type window layer 4 are epitaxially grown on an n-type substrate 2 in this order. The top surface is masked with a resist layer and then treated by photolithography to form openings arranged one-dimensionally with a constant pitch. A p-type dopant is diffused from above through the openings to form a plurality of p-regions 5. At the same time, pn junctions 6 are formed in the window layer 4 and the absorption layer 3, as shown in FIG. 1. A p-electrode 7 is bonded with ohmic contact to a part of the top surface of each of the p-regions 5. The top faces of the p-regions 5, except the portions for the p-electrodes 7, and the n-type window layer 4 are covered with an antireflective coating 9. A common n-electrode 8 is formed at the rear face of the n-type substrate 2.
FIG. 1 shows an array composed of four unit photodiodes, PD1, PD2, PD3, and PD4. Actually, an array having a different number of unit photodiodes may be used depending on the application, the number being a power of two such as eight or 16. In a practical application, the chip shown in FIG. 1 is mounted on a package and the p-electrodes are wire-bonded with lead pins. Finally, they are sealed to form a completed product. In FIG. 1, the package and the bonding wires are not shown. In the optical communication use, the n-type substrate made of InP is used. In the case of the InP substrate, the absorption layer made of InGaAs in the form of a ternary mixed crystal and the window layer made of InP are used. In many cases, Zn is used as the p-type dopant. The photodiode is classified as an InP-based one, an Si-based one, a GaAs-based one, and so on according to the type of the substrate.
In this example, four signal lightwaves λ1, λ2, λ3, and λ4, each having a different wavelength, enter the corresponding photodiode units PD1, PD2, PD3, and PD4 from above. These signal lightwaves are introduced into this place through an optical fiber, and each signal lightwave is condensed at the surface of a photodiode unit with a condenser lens. The signal lightwave having entered from above produces electron-hole pairs at a depletion layer extending at both sides of the pn junction.
An electric field applied to the depletion layer drives a minority carrier to the pn junction, and then the carrier crosses the pn junction and becomes a majority carrier, producing a photocurrent at the same time. At this moment, when the photodiode array has a continuous structure as shown in FIG. 1, electrical crosstalk may occur. The reason is that because the n-type window layer 4 has an electrical conductivity to a certain extent, a leakage current flows across neighboring photodiode units.
To overcome this problem, a photodiode array as shown in FIG. 2 has been proposed, in which individual photodiodes are isolated by isolation grooves 22. An undoped absorption layer 3 and a p-region 5 are epitaxially grown on an n-type substrate 2. P-electrodes 7 and antireflective coatings 9 are provided on the p-region 5. To isolate the photodiodes, the isolation grooves 22 are formed vertically by etching. Thus, the photodiode units PD1, PD2, PD3, and PD4 are electrically isolated. This structure is expected to prevent the current from flowing across the neighboring photodiodes because the portions of the p-regions are completely isolated. In practical applications, when the inside of the isolation grooves 22 is empty, the pn junction may deteriorate or other problems may be caused. To prevent these problems, the isolation grooves 22 are filled with some material.
The published Japanese patent application Tokukai 2001-144278 has proposed an InP-based photodiode array provided with isolation grooves as shown in FIG. 2. The isolation grooves 22 isolate neighboring unit photodiodes. The side wall of the isolation groove is coated with an insulation film, made of SiN or another material, formed by CVD or another proper method. Thus, the pn junction is protected.
Another published Japanese patent application, Tokukai 2001-352094, has proposed an improvement of an Si-based photodiode array. To isolate photodiodes in the Si-based photodiode array, isolation grooves are formed by etching and subsequently filled with a filling material made of SiO2. This structure is intended to prevent electrical crosstalk between neighboring photodiode units.
In an array in which a plurality of photodiodes are arranged two- or one-dimensionally, various methods are devised to suppress electrical crosstalk between individual photodiodes. An incoming lightwave is introduced through an optical fiber, is condensed with a lens, and enters the unit photodiode perpendicularly in many cases. Consequently, it has been considered that no optical crosstalk occurs. As a result, no conventional designs take the optical crosstalk into account. In actual fact, however, optical crosstalk occurs between neighboring photodiode units.
The optical crosstalk between neighboring photodiode units is explained below by referring to FIG. 3. First, a signal lightwave λ1 enters PD1 at the left-hand side. Although the lightwave is condensed with a condenser lens after emerging from an optical fiber, some components of the lightwave enter the photodiode obliquely to a certain extent. Nearly all of them are absorbed in the absorption layer 3 and produce electron-hole pairs at the depletion layer near the pn junction. Carriers driven by the electric field toward the pn junction cross the pn junction and produce a photocurrent.
In this case, part of them pass through the absorption layer 3 because the layer is thin. The leakage lightwave λ1 having transmitted passes through the n-type substrate 2 and hits the n-electrode 8 at the rear side. Because the n-electrode 8 is metal, the leakage lightwave is reflected from the boundary surface between the n-electrode 8 and the substrate. The reflected lightwave λ1 is absorbed again in the absorption layer 3. In other words, the remaining slight components arrive at the pn junction 6 of the neighboring photodiode and produce electron-hole pairs there, causing PD 2 to produce a photocurrent. The lightwave that has produced the photocurrent is not the intended lightwave entering PD2 from above but the leakage lightwave from the signal lightwave having entered PD1. Because the leakage lightwave from PD1 enters PD2, this phenomenon is optical crosstalk.
Such optical crosstalk cannot be prevented by an isolation groove provided between photodiodes. FIG. 4 shows two unit sections of a photodiode array having isolation grooves (see the foregoing Tokukai 2001-144278 and Tokukai 2001-352094). Nearly all of the components of the lightwave λ1 having entered PD1 are absorbed in the absorption layer 3 and converted into a photocurrent of PD1. However, some of the components pass through the absorption layer 3 and the substrate 2. They are reflected from the n-electrode 8 at the rear side, travel in the substrate 2 upward, and enter the absorption layer 3 and the pn junction 6 of the neighboring PD2. They cause PD2 to produce an additional photocurrent. Even the isolation groove 22 is provided, the reflected lightwave passes through the substrate 2 without being blocked by the isolation groove 22. In other words, the isolation groove 22 cannot prevent the optical crosstalk.
The leakage lightwave may reach not only the neighboring photodiode unit PD2 but also remotely located photodiode units PD3, PD4, and so on. To achieve ohmic contact with the n-type substrate 2, the n-electrode 8 is processed by alloying treatment at high temperature. The heating causes atoms of the n-electrode and the substrate to mutually diffuse. As a result, the boundary surface between the substrate and the electrode is not smooth but undulating. This undulating boundary surface irregularly reflects the leakage lightwave. Consequently, the leakage lightwave may reach photodiode units placed in a considerably remote location. This is the reason why the optical crosstalk is caused.